At first, description will be made about an organic electroluminescence (ElectroLuminescence: EL) element and single-wall carbon nanotubes (Single-Wall carbon NanoTubes: abbreviated to SWNTs) material, both of which are related to this invention.
First, the organic EL element is a kind of light-emitting diode driven by a direct current and is also called an organic electroluminescence element and an organic LED (Light-Emitting Diode). At any rate, the most basic structure of the organic EL element mostly is a sandwich structure such that a light-emitting layer (might be called a luminescent layer) formed of an organic compound is sandwiched by two electrodes, i.e., a cathode and an anode. A wide variety of proposals have been made about the light-emitting layer ranging from a single-layer structure to a multilayer structure. Depending on circumstances, each layer of the respective structures has a different function. In general, a hole-injection layer and a transport layer are laminated on an anode side while an electron-injection layer and a transport layer are laminated on a cathode side.
As regards the electrodes of the EL element, a transparent electrode is used as one of the electrodes so that light of the light-emitting layer can be extracted or emitted. In most cases, for the anode, use is made of a transparent metallic material which has a comparatively high work function and which is called ITO (Indium Tin Oxide, tin-doped indium oxide) or IZO (Indium Zinc Oxide, zinc-doped indium oxide).
According to Patent Document 1 (JP-B-2597377 (page 14, FIG. 1)), an anode material desirably has a work function of not less than 4 eV.
On the other hand, for the cathode, use is made of metal, such as calcium (Ca), aluminum (Al), and magnesium-silver (MgAg), which has a comparatively low work function. For a property of a cathode material, it is required to have a work function of not more than 4 eV, for example, as described in Patent Document 1.
An operating mechanism of the organic EL element is described in a number of documents. For example, the mechanism is described in Non-patent Document 1 (“Year of Heisei 17 [2005], Patent Application Technology Trend Research Report, Organic EL Element (abridged edition)”, edited by the Japan Patent Office (the technology trend team of the technology research division of the general affairs department of the Japan Patent Office), April in Heisei 18 [2006], pp. 1-2). According to the report, in an organic EL element having the above-mentioned structure, electrons and holes are injected from a cathode and an anode, respectively, to an organic light-emitting layer. In the organic light-emitting layer, the electrons and the holes are recombined so that organic molecules are put into an excited state. A light-emitting excitation is converted into light specific to the molecules. Efficient light emission of the organic EL element requires improvement of an injection efficiency and a charge balance of the holes and the electrons. For this purpose, in a related art, the hole-injection layer, the electron-injection layer, and a buffer layer are arranged between the electrodes and the light-emitting layer.
Next, description will be made about SWNTs material and doping therefor.
SWNTs have a cylindrical graphite structure having a diameter of several nanometers and a length between several hundred nanometers and several micrometers. Depending on the chirality and the diameter of the SWNTs, there are metallic SWNTs and semiconductor SWNTs. The semiconductor SWNTs can be used for a channel of a transistor. As compared to silicon, the semiconductor SWNTs have a ten times or more drift mobility and a band gap can be structurally controlled by its diameter and chirality. For the above-mentioned reasons, the semiconductor SWNTs are would be considered as particularly important in device application as a post-silicon semiconductor material.
In general, doping means addition of a foreign substance in order to mainly control the property of a semiconductor and particularly to control a conduction type of the semiconductor. There are two conduction types of the semiconductor, i.e., n-type conduction and p-type conduction. A semiconductor exhibiting the n-type conduction is called an n-type semiconductor and electrons contribute to electric conduction. The electrons are supplied from a donor (an electron donor, an n-type dopant) as the foreign substance to a conduction band of the semiconductor. A semiconductor exhibiting the p-type conduction is called a p-type semiconductor and holes contribute to electric conduction. The holes are generated as a result of electrons being taken from a valence band of the semiconductor and captured by an acceptor (an electron acceptor, a p-type dopant) as the foreign substance.
As for the SWNTs, by doping suitable donors or acceptors, n-type conduction SWNTs or p-type conduction SWNTs are produced.
For example, as a related art of producing the n-type conduction SWNTs, the following methods have been reported. Non-patent Document 2 (Physical Review B, Vol. 61, pp. R10606-10608, 2000) discloses a method of vapor-depositing potassium (K) and Non-patent Document 3 (Physical Review Letters, Vol. 87, pp. 256805-256808, 2001) discloses a heat treatment in a vacuum. However, SWNTs channels formed by both of the above-mentioned methods are not suitable for fabrication of a device that is stably operable. This is because the SWNTs channel is chemically unstable in the atmosphere since the donor exists outside SWNTs walls.
As other related arts of producing the n-type conduction SWNTs, there are proposed the following methods. Non-patent Document 4 (Journal of American Chemical Society, Vol. 123, pp. 11512-11513, 2001) discloses a method of supplying polymer containing an imine group from outside the SWNTs. In addition, Patent Document 2 (JP-A-2004-311733 (p. 9, FIGS. 1 and 3)) discloses a method of introducing organic molecules to be a donor into a SWNTs cavity. The n-type conduction SWNTs produced by those methods are stable in the atmosphere but are low in conduction controllability, such as a carrier density, because of use of an organic material. This is because the organic material has a higher ionization potential as compared to alkali metal and the like and therefore a charge amount induced within the SWNTs is relatively small.
Furthermore, as a related art of producing the p-type conduction SWNTs, a method has been reported in which, without performing any special treatment to the SWNTs, oxygen molecules and water molecules that would be considered as a hole supply source are spontaneously adsorbed from the atmosphere. However, with this method, characteristics of a SWNTs transistor are changed depending on an external environment. Therefore, it is not possible to manufacture a reliable device.
Moreover, as another related art of producing the p-type conduction SWNTs, Patent Document 2 has proposed a method of introducing or encapsulating organic molecules acting as acceptors into the SWNTs cavity. The p-type conduction SWNTs produced by this encapsulation are stable in the atmosphere. However, because an encapsulated organic material has a low electron affinity (approximately 3 eV at most) in comparison with an electron affinity of an inorganic material, for example, an electron affinity (approximately 8.4 eV) of tantalum fluoride, the conduction controllability, such as a carrier density, is low.
Furthermore, Patent Document 3 (JP-A-2006-190815 (pp. 4 to 5)) describes details of a principle that the work function of the SWNTs is controlled based on charge transfer caused when donors or acceptors are arranged on a surface of the SWNTs, and the SWNTs encapsulating the donors or the acceptors functions as an electrode controlled in work function. Herein, the principle described in Patent Document 3 will briefly be described. Bands, such as a valence band and a conduction band, in the vicinities of the surfaces of the SWNTs are bent by the charge transfer from the donors or to the acceptors so as to raise or lower a position of a Fermi energy level in the vicinities of the surfaces of the SWNTs. In other words, this shows that a value of the work function of the SWNTs is relatively changed. When a donor having an ionization potential (IP) lower than the work function of the SWNTs is arranged, the band of the SWNTs is bent in an energy depth direction, namely, downward. With this bending, the Fermi energy level of the SWNTs is relatively raised and the work function of the SWNTs is decreased. On the other hand, when an acceptor having an electron affinity (EA) greater than the work function of the SWNTs is arranged, the band of the SWNTs is bent upward. With this bending, the Fermi energy level of the SWNTs is relatively lowered and the work function is increased. As the IP of the donor arranged is smaller or as the EA of the acceptor is greater, work function shift of the SWNTs becomes greater. Further, as a surface density of the donors or the acceptors is greater, the work function of the SWNTs is greatly changed. Therefore, by controlling a kind and a concentration of the donors or the acceptors, SWNTs having a desired work function value can be obtained.
Next, description will be made as regards an operation of a work function controlled SWNTs electrode for the purpose of achieving improvement of luminance efficiency and low power consumption of an EL element.
An EL element according to a related art has a structure in which a light-emitting layer is sandwiched between ITO having a high work function (work function: W=4.7 to 5.2 eV) and serving as a positive electrode or anode, and alkali metal or alkaline earth metal having a low work function and serving as a negative electrode or cathode, for example, calcium: Ca (W=2.9 eV). An electronic structure of the EL element is shown in schematic diagrams of FIG. 10. In FIG. 10, (A-1) to (A-3) show changes in energy level when a bias voltage: Vbias is gradually applied. (A-1) shows a zero-bias state (Vbias=0V) where Fermi energy levels of the both electrodes are coincident with each other. (A-2) shows a flat band state where a bias voltage is applied by a work function difference: ΔW between the both electrodes (in this case, Vbias=ΔW=1.8 eV). (A-3) shows a state where an electric voltage is further applied and holes and electrons are recombined with each other at the light-emitting layer by current injection to emit light (in this case, Vbias>ΔW=1.8 eV). If there is no injection barrier at interfaces between the light-emitting layer and the electrodes (anode side: φBp, cathode side: φBn), light emission starts from the flat band state in (A-2). Actually, however, light emission does not start unless Vbias significantly exceeds ΔW because of presence of φBp and φBn having finite heights. In general, because φBp and φBn are different in magnitude, hole injection and electron injection do not balance with each other. This results in flowing of useless current which does not contribute to light emission without occurrence of recombination.